An explosion in a police compound in Turkey’s southeastern city of Diyarbakir killed one person and wounded several others on Tuesday, days ahead of a national referendum, but the interior minister appeared to rule out foul play.

The blast, which caused part of the police compound’s roof to collapse, occurred during the repair of an armoured vehicle, the Diyarbakir governor’s office said. It said one person had died in hospital and the cause of the blast was unknown.

Diyarbakir is the largest city in Turkey’s southeast, where Kurdish PKK militants have fought an insurgency for more than three decades to press demands for Kurdish autonomy. Violence has flared since a ceasefire collapsed in July 2015.

But Interior Minister Suleyman Soylu said the explosion, which sent a large plume of smoke over surrounding buildings and left a crater in the ground, appeared to have been an accident.

“The blast was in a part of the building for riot police, where maintenance is carried out on vehicles,” he was quoted by broadcaster CNN Turk as saying during a speech in Istanbul.

“At the moment, it seems there is no outside interference, and the explosion came from the vehicle under repair. One person is trapped under the wreckage,” he said.

The explosion came ahead of a hotly contested referendum on Sunday on broadening President Tayyip Erdogan’s powers, a constitutional change opposed by many in Turkey’s predominantly Kurdish southeast.

The blast was in the central, largely residential district of Baglar, where a car bombing by suspected PKK militants wounded scores of people last November.